The invention relates to oxidized starch, the production thereof, as well as to the use of oxidized starch in various applications.
Oxidized starches have found many applications in industry. Examples of common applications include the use of oxidized starch in the paper industry, for instance in coatings or surface sizing, the adhesive industry, the textile industry, and the food industry.
The preparation of oxidized starches is conventionally carried out by oxidation with an alkali metal hypochlorite, which is a relatively cheap oxidizing agent.
The main factors controlling the oxidation reaction are the amount of alkali metal hypochlorite used, the pH, the temperature, and the use of metal and/or bromide ions as catalyst. An overview of the most important reaction parameters may be found in an article by J. Potze and P. Hiemstra in Starch, vol. 15, pp. 217-225 (1963). It has been proposed that dissociation of the hypochlorite in solution and the presence of radicals in the reaction mixture determine the reaction mechanism. Despite the extensive research done, the exact mechanism of the hypochlorite oxidation of starch is, however, still not entirely clarified.
As has been mentioned above, the course of the oxidation reaction using an alkali metal hypochlorite depends much on the pH during the reaction. This dependency has been widely addressed in the literature. The highest reaction rates are found at neutral pH, while the reaction rate decreases with increasing pH. At acidic pH ( less than 5), chlorine is formed, which, for evident reasons, is to be avoided in an industrial process. Thus, from the view of the reaction rate, it would be desirable to perform the oxidation reaction at or around a neutral pH.
During the oxidation of starch with an alkali metal hypochlorite, different reactions occur. These reactions lead to the introduction of carboxyl and carbonyl groups, and to the degradation of the starch molecule. The course of all these reactions, and the balance among them, determine the properties of the oxidized starch that is obtained. The balance among said reactions, i.e. the relative amount in the oxidized starch of carboxyl and carbonyl groups and the extent of degradation of the starch molecule, are, in their turn, dependent on the pH during the oxidation reaction. Hence, the properties of an oxidized starch will depend on the pH at which the oxidation reaction is carried out.
The degradation of the starch molecule during oxidation leads to a lower viscosity of a solution (or dispersion) of the oxidized starch, which is usually desired of an oxidized starch. It has been found that the degradation occurs to a farther extent at neutral pH of about 7-7.5, than at alkaline pH, such as pH 9 or higher. In other words, in order to obtain an optimal yield of oxidized starch providing a solution or dispersion of low viscosity, the oxidation reaction should preferably be carried out at neutral pH.
However, the viscosity in solution (or dispersion) is not the only important property of an oxidized starch. The viscosity of said solution (or dispersion) is, for most purposes, required to not, or hardly, fluctuate in time. The viscosity of the solution (or dispersion) should remain stable during storage.
Carboxyl groups, that may be introduced in the starch during the oxidation reaction, provide the desired stability of the viscosity of an oxidized starch solution or dispersion. The higher number of carboxyl groups, the better the viscosity stability. Contrary to the degradation of the starch, the amount of carboxyl groups introduced in the starch during oxidation with an alkali metal hypochlorite is small when the oxidation is carried out at neutral pH. The pH at which a high number of carboxyl groups is introduced lies around 8.5.
Besides choosing the oxidation reaction conditions so that a high number of carboxyl groups are introduced, the stability of the viscosity of solutions (or dispersions) of oxidized starch may be increased by introducing ether or ester groups in the starch. Examples of such groups include hydroxyethyl, hydroxypropyl and acetyl groups. Disadvantages of this route are that an additional derivatization step is needed, in which toxic reagents are used.
The number of carbonyl groups introduced in the starch during oxidation negatively affects the stability of the viscosity of the oxidized starch in solution (or dispersion). It further leads to a more brown-yellow color of a solution or dispersion of the oxidized starch, which is usually not wanted. The amount of carbonyl groups introduced is also dependent on the pH during the oxidation reaction. At neutral pH, a relatively high number of carbonyl groups is introduced. At a higher pH, less carbonyl groups will be introduced during oxidation.
From the above, it will be clear that the choice for the pH at which conventional oxidation reactions of starch using an alkali metal hypochlorite are performed, constitutes a compromise between efficient starch degradation and stability of the viscosity of the oxidized starch when dissolved or dispersed. This compromise becomes even more apparent from the data presented in the below table I.
In table I, the number of+""s indicates the extent to which the specific reaction occurs at a given pH.
Conventionally, the pH during oxidation of starch using an alkali metal hypochlorite is chosen at 8.5 or higher, dependent mostly on the desired viscosity of the oxidized starch.
The international patent application WO-A-97/04167 relates to the use of an amylopectin type starch, obtained from potato that has been modified by genetic engineering to suppress the formation of amylose-type starch, as a finishing agent in papermaking. The amylopectin type starch is subjected to oxidation, enzymatic treatment, acid hydrolysis or thermo-chemical conversion. The disclosed oxidation process is performed at pH 9.5. According to the document, the obtained oxidation product may, as such, be used as a finishing agent in papermaking.
As has been indicated above, for reasons of stability, in particular viscosity stability, of the oxidized starch, the oxidation process is usually carried out at a pH higher than 8.5. However, the high pH adversely affects the reaction rate. Also, carrying out the oxidation process at this pH has the effect that relatively high amounts of oxidizing agent are necessary to achieve the desired viscosity. As the oxidizing agent is usually an alkali metal hypochlorite, the more oxidizing agent is used, the higher the risk that a certain amount of chlorine ends up in the oxidation product. The presence of chlorine is for evident reasons highly undesirable with respect to (public) health and the environment.
The present invention aims to overcome the above problems. The invention further aims to provide a process for the oxidation of starch wherein significantly less oxidizing agent is necessary than in the prior art processes. It is also an object of the invention to provide a process for the oxidation of starch wherein an oxidized starch is obtained, which has excellent properties, such as viscosity and (viscosity) stability.